1. Field of the Invention
This invention relates to a vibration/noise control system, and more particularly to a vibration/noise control system adapted to actively control vibrations and noises with a periodicity or a quasi-periodicity generated from a rotating member and the like, for reduction thereof.
2. Prior Art
Recently, active vibration/noise control systems have been developed in various fields of the industry, which are adapted to damp vibrations and noises produced from vibration/noise sources by the use of an adaptive digital filter (hereinafter referred to as an "ADF") to thereby reduce the vibrations and noises.
One of the conventional active vibration/noise control systems of various types is a vibration/noise control system proposed by the present assignee, which is suitable for reducing vibrations and noises generated from an engine of an automotive vehicle and the like with a periodicity or a quasi-periodicity (Japanese Patent Application No. 4-88075, which is incorporated in U.S. Ser. No. 08/029,909, now U.S. Pat. No. 5,386,372, and hereinafter referred to as "the first prior art"). This system comprises an adaptive control circuit supplied with a predetermined pulse signal (trigger signal) related to driving of a power plant, and first filter means comprised of an ADF for adaptive control of the vibrations and noises.
According to the first prior art, the pulse signal is directly supplied to the adaptive control circuit, which makes it possible to reduce the number of complicated product-sum operations to thereby enhance a converging speed of the adaptive control for reducing the vibrations and noises. Further, the pulse signal is input to the adaptive control circuit at proper time intervals dependent on operating conditions of the engine for execution of the adaptive control dependent on the proper time intervals. This makes it possible to perform the vibration/noise control with high accuracy. Further, according to the first prior art, the sampling repetition period is varied depending on timing of operation of each pulse of the pulse signal, and hence even for a power plant which produces vibrations and noises having waveforms changing largely due to changes in the rotational speed of an engine thereof, the sampling repetition period can be varied according to the changes in the rotational speed of the engine, which makes it possible to attain an increased speed of follow-up in control, and hence to perform the adaptive control with high accuracy.
Further, an active vibration control system which uses a sine wave signal as a reference signal to be input to an ADF has already been proposed by International Publication No. W088/02912 (hereinafter referred to as "the second prior art"), which counts pulses of a pulse sequence signal related to the rotational speed of an engine, and generates the sine wave signal in synchronism with a predetermined clock pulse signal.
The second prior art counts pulses of the pulse sequence signal at a constant sampling frequency based on the predetermined clock pulse signal to thereby generate two predetermined trigonometric functions, and then synthesizes these trigonometric functions by the use of an oscillator into the sine wave signal of a digital type.
Further, a vibration control system which is adapted to perform the adaptive control based on a signal sampled in synchronism with the rotation of the engine has been proposed e.g. by International Publication No. W090/13108 (hereinafter referred to as "the third prior art"), which subjects an error signal to an orthogonal transformation, such as Discrete Fourier Transform (DFT), to control vibrations and noises peculiar to respective component parts of the engine, independently of changes in the rotational speed of the engine.
The third prior art subjects waveforms of vibrations and noises peculiar to respective component parts of the engine to the orthogonal transformation to deliver control signals prepared by filtering of the waveforms of vibrations and noises for control of the vibrations and noises as desired.
However, in the first prior art proposed by the present assignee, the reference signal input to the ADF is the pulse signal, and hence the ADF is required to have a tap length adaptable to all variations of the reference signal. Further, depending on the repetition period of vibrations and noises, the tap length can become so long that the product-sum operation (convolution) takes much time to lower the converging speed of the adaptive control.
Further, in the first prior art, the adaptive control circuit is provided with second filter means for correcting changes in phase, amplitude, etc. of the control signal caused by the transfer characteristic (transfer function) of a path through which the vibrations and noises are transmitted, and filter coefficients of the first filter means are updated taking a second reference signal output from the second filter means. However, a proper value of the transfer function of the path varies with periodicity of the reference signal (pulse signal) input, and hence when the sampling frequency, which is dependent on the timing of inputting of the reference signal, undergoes a change, it is required to change the filter coefficients of the second filter means representative of the transfer characteristic (transfer function) of the path according to the changes in the sampling frequency. This complicates the computing processings.
In the second prior art, the two trigonometric functions are synthesized by the oscillator into the digital sine wave signal. The synthesis of the sine wave signal takes much time. Further, when the count of clock pulses is deviated from a proper value, a spike (a phenomenon of generation of a distortion in the form of a pulse waveform of a very short duration relative to the pulse width) and jitter (a phenomenon of the pulse width being instable) can occur.
Further, in the second prior art, even if the sine wave signal is used for the reference signal, the filter means representative of the transfer characteristic of the path is required for each of the frequency components of vibrations and noise. This increases the tap length (number of filter coefficients) of the filter means and hence the processing takes much time to degrade the convergence of the adaptive control. Therefore, there can be a case in which the system cannot follow up changes in the rotational speed of the engine.
Further, in the third prior art, to make the system adaptable to changes in the sampling frequency dependent on the periodicity of vibrations and noises generated from various sources, it is required to store in advance filter means representative of transfer characteristics of the path by the use of a large number of storage elements, or alternatively store in advance a small number of filter means representative of the transfer characteristics, and then set proper filter means by interpolation based on the stored filter means according to the frequency components to allow them to properly represent the transfer characteristics of the path. Therefore, it is either required to use a lot of storage elements, or to spare much time for the processing.